Articles manufactured from poly(1,4-butylene terephthalate) have many valuable characteristics, including strength, toughness, solvent resistance, high gloss, and the like. These articles may be fabricated by a number of well-known techniques, including injection molding, roto molding, blow molding, extrusion, and the like, depending on the shape of the desired product.
Certain of these techniques, in particular, blow molding and extrusion, require that the molten poly(1,4-butylene terephthalate) have a suitably high melt viscosity, e.g., in excess of 10,000 poises, to prevent collapse or blow-outs in the soft preformed state. It has been found that poly(1,4-butylene terephthalates) of such high melt viscosity are obtained only with great difficulty in the conventional bulk melt polymerization processes generally used to prepare the polyester. This is probably because 1,4-butanediol must be eliminated to increase molecular weight and the last traces are difficult to remove from the hot, viscous melt. In most cases, the intrinsic viscosity (measured at 30.degree.C. in a 60:40 mixture of phenol and tetrachloroethane) reaches a level of about 0.2 to 1.2 deciliters per gram and then stabilizes, with 0.85-1.05 dl./g. generally being the practical maximum. In addition, polymer melts with a melt viscosity greater than 10,000 poises become increasingly difficult to handle and to remove from commonly used bulk polymerization equipment, leading to degradation and discoloration of the products.
Solid-state polymerization is a known technique for increasing the intrinsic viscosity of poly(ethylene terephthalate). This gives a polyester which will produce a higher-tenacity industrial yarn. Thus, in U.K. Pat. No. 1,066,162 it is disclosed that when this linear polyester is heated at temperatures below its point of thermal decomposition, in a vacuum, coupling of the polymer chains produce increases in molecular weight.
However, it is also known that under such conditions, undesirable depolymerization by ester interchange is equally favored with the desired chain coupling molecular weight increase. H. A. Pohl, J. Am. Chem. Soc., 73, 5660 (1951) has studied th relative rates of decomposition and reported that the terephthalate polyester of ethylene glycol was much more stable than the polyester with decamethylene glycol. In other words, the decamethylene glycol terephthalate polymer decomposes on heating by evolving gas, changing color and liberatig acid much more readily than the corresponding ethylene glycol terephthalate polymer.
In copending application Ser. No. 415,161, it has been disclosed that poly(butylene terephthalate) with a very high melt viscosity (corresponding to an intrinsic viscosity of greater than about 1.05 deciliters per gram) can be obtained by heating a solid, particulate form of poly(1,4-butylene terephthalate) of lower intrinsic viscosity, e.g., 0.7-1.0 dl./g. at a temperature of about 150.degree.C. and below the melting or sticking point of the particles, for a period of time varying from 1/2 hour to several days, or longer, depending on the temperature, and the intrinsic viscosity desired. The poly(1,4-butylene terephtaalate) will also preferably include small quantities of branching agents.
It is most unexpected to find that such a polyester, which has a higher number of carbon atoms in the alkylene units than in poly(ethylene terephthalate), can be heated without decomposition in view of the teachins of Pohl. Moreover, another surprising difference between solid-state polymerization of poly(1,4-butylene terephthalate) in comparison with poly(ethylene terephthalate) is the high rate at which the increase in intrinsic viscosity occurs with the former at relatively low reaction temperatures and at atmospheric pressure, instead of under high vacuum.
When the solid particulate form of poly(1,4-butylene terephthalate) is polymerized according to the above-disclosed method, it has been found that the polymer molecules on the outer surfaces of the individual particles polymerize at a higher rate than the polymer which is in the interior of the individual particles due to the greater ease with which the polymerization by-product glycol is removed from the surface of the cubes. As a result the "skin" of the individual particles tends to gel before the bulk of the mass has polymerized to the desired extent. This is disadvantageous as the presence of insoluble and infusible gel particles in a thermoplastic material is undesirable, since they interfere with normal processing procedures, and affect the properties and appearance of molded articles. This is sometimes a particularly serious problem with branched copolyesters.
Therefore, it is an object of this invention to provide a controlled solid-state polymerization process for poly(1,4-butylene terephthalate) which proceeds at a controlled rate.
It is also an object of this invention to provide a solid state polymerization process which may be employed to produce higher molecular weight poly(1,4-butylene terephthalate).
These and other objects of the invention will be readily apparent from a reading of the following description of the invention and description of the preferred embodiments.